In the production of zinc metal employing the roast-leach-electro-winning method, thallium ions in ZnSO.sub.4 solution deposit preferentially over zinc ions during the electrolysis, thus contaminating the zinc metal recovered at the cathodes. Therefore, it is mandatory to keep the concentration of thallium ions in the electrolyte at levels as low as possible. In general, the low concentrations of thallium are achieved in the solution purification step, in which zinc metal powder is used to cement out dissolved elements nobler than zinc. Though this is the commonly used method in the zinc industry, very narrow operational conditions are required to achieve efficient thallium removal.
Residues produced in the zinc dust purification step contain valuable metals like copper, cadmium, cobalt and nickel, and are generally further processed to recover these metals. During the recovery process, the thallium present therein redissolves and remains in the leach solution, which are often recycled to the leach plant. Thus, thallium remains in the closed loop leach circuit, which necessitates a bleed for the thallium from the circuit.
In the zinc industry, the removal of thallium ion is effected by addition of potassium permanganate, potassium dichromate or potassium chromate in the streams where cadmium metal is recovered. The addition of potassium permanganate converts thallous (Tl.sup.+1) ions to thallic (Tl.sup.+3) ions, the latter being insoluble at pH values above 3, thus precipitating as Tl(OH)3. This process cannot be used economically for solutions containing high concentrations of ionic species which are oxidized with the permanganate ion. Manganese ion is a good example. The presence of manganese ion in solution consumes permanganate ions before thallous ions are oxidized, resulting in high consumption rate of potassium permanganate, which is an extremely expensive reagent.
On the other hand, the addition of potassium dichromate or potassium chromate to solutions containing thallium results in the formation of thallous dichromate and thallous chromate, which are relatively insoluble at ambient temperature, and precipitate. Since the thallous dichromate/chromate solubility increases with temperature, the solution must be kept at lower temperatures to enhance the efficiency of the process. Also the process introduces chromium, an undesirable element, in the zinc solution circuit, necessitating an additional step for removing chromium if no outlet for this element exists in the circuit. Again, the economical aspect of this process is questionable, since potassium dichromate and potassium chromate are expensive reagents. Further, they introduce a new metal, chromium, which compel one to remove this metal from the solution.
Ozone and persulphate compounds are also known to convert thallous (Tl.sup.+1) ions to thallic (Tl.sup.+3) ions and to precipitate the thallic hydroxide (Tl(OH).sub.3). However, like the preceding reagents, these are also very strong oxidizers, and oxidize other elements that may be present in the aqueous solution, for example chloride, nickel, cobalt and manganese. This process, like the others discussed above, is limited to solutions containing low concentrations of elements susceptible of being oxidized by the reagents.
Recently, ion exchange resins were found to remove thallium ion in aqueous streams (see AU 634,853). However, this technology has not been exploited on a commercial scale.
There is therefore a great need to develop a method for effectively removing thallium ions present in aqueous solutions without affecting the other elements or metal ions present in the solution with minimum reagent consumption. Further, such method would use an inexpensive reagent, and both the method and the reagent should be environmentally friendly.